This invention relates generally to structures and techniques for fabricating and isolating microelectronic devices and, in particular, to microelectronic devices and processes including trench isolation with stress relief.
In the fabrication of microelectronic circuits it is necessary to isolate one or more microelectronic transistors from another. While there are a number of techniques available for providing such isolation, trench isolation is often used for separation between adjacent device islands. Trench isolation involves etching a narrow, deep groove in the substrate, usually silicon. After etching the groove, the inside surface of the groove is usually filled with an oxide or polysilicon, or both.
In a typical trench isolation process, a layer of masking material is deposited or grown on the substrate and etched to form openings defining a pattern of the length and width of a trench. The substrate is then anisotropically etched to remove silicon. After etching is completed a sidewall oxide is grown on the inside surface of the trench and then the trench is filled with polysilicon by deposition and etchback. As a final step, an oxide cap is grown on the trench. The trench masking material is typically removed prior to the growth of the oxide cap or prior to the formation of the sidewall oxide.
The trench is made deep enough to penetrate epitaxial layers in the substrate. This penetration will, in effect, decouple parasitic bipolar transistors that are inherently formed during a CMOS process. Alternatively, the trench may be made deep enough to reach a buried oxide in an SOI material.
As part of a quality assurance program, wafers containing trench isolation were etched to identify any defects associated with trench location. As a result of this investigation, we uncovered, in the worst samples, unusual behavior. In certain samples, device islands were relatively free of defects whereas certain field areas where no devices were formed had high defect concentration.
More specifically, we noticed that in large square islands such as bond pads that contained no diffusions, there were many defects adjacent the trench area, in particular, near the corners of the trench. The field area outside the bond pads had numerous defects some of which emanate from outside comers. In all of the cases, the defects appeared to be aligned with one of the trench lines entering a comer. Indeed, the defects appear to be distributed along an imaginary line that would be a continuation of the trench line.